In “The Apartment” episode of “Mad About You,” which aired in 1992 Paul (Paul Reiser) signs over the lease of his old place to the current tenant — Cosmo Kramer (Michael Richards). The two guys celebrate with a cigar in the hallway. Paul asks whatever happened to that Jerry guy across the hall. “He’s writing a sitcom for NBC,” Kramer says. Paul rolls his eyes and replies, “Good luck to him.”

2. Happy Days, Mork & Mindy and Laverne & Shirley

“Happy Days” inspired two spin-offs, “Laverne & Shirley” and “Mork & Mindy.” Mork made two appearances on “Happy Days” and the three shows joined forces for the 1978 debut of “Mork & Mindy.” While Fonzie (Henry Winkler) is house-sitting for the Cunninghams, Mork (Robin Williams) shows up in his space costume and Fonzie sets him up with Laverne (Penny Marshall).

“Is it time to flatter you?” Mork asks. “Sure,” says Laverne. Seeing the flower in her hair, he says, “You have a lovely fungus growing out of your head.”

3. Cheers and St. Elsewhere

A 1985 episode of “St. Elsewhere” featured a “Cheers” crossover, in which Drs. Westphall (Ed Flanders), Auschlander (Norman Lloyd) and Craig (William Daniels) stop into the pub for a drink, and Craig gets into an argument with Carla (Rhea Perlman) who had delivered her most recent child at St. Eligius and hated the place.

4. Murphy Brown and The Bob Newhart Show

Murphy Brown (Candice Bergen) could never keep an assistant, until she met Carol (Marcia Wallace), Bob Newhart’s girl. Newhart, whose show had been off the air, rushed in at the end to convince Carol to come back to him.

5. Bewitched and The Flintstones

In 1965, Samantha and Darrin Stephens (Elizabeth Montgomery and Dick York) visited Bedrock as Fred and Wilma’s new neighbors. When Fred and Barney go on a camping trip without their wives, Samantha works her magic in helping Wilma and Betty follow the men and have their own fun in the woods. Montgomery and York did the voices of their characters.